Reproduction and feather molt are two of the most energetically demanding aspects of birds' lives. The breeding season can last five months or longer, and feather molt can take six to 10 weeks. Just finding enough food to stay alive during these stressful times can be a full-time job.
Migratory species face additional challenges. Spring and fall migration can take weeks to complete, but migrants also face dangers ranging from aerial predators to severe weather. A recent report from the Dauphin Island Sea Lab in Alabama revealed an additional and totally unexpected danger for land birds crossing the Gulf of Mexico.
Sharks in Gulf waters eat a surprising number of land birds, everything from swallows and kingbirds to tanagers and catbirds. Feather balls, analogous to fur balls that cats cough up, have been found in shark stomachs during unrelated research. Some of these feather balls are as big as grapefruits.
These findings prompted researcher Dr. Marcus Drymon to wonder, "How do land birds end up in the water as food for sharks? This is certainly not something we expected to see."
"Bird migrations across the Gulf are incredibly strenuous treks that result in large numbers of bird deaths over water from exhaustion, but there may be other factors at play here," said Drymon. "We're going to take a closer look at this over the next year and see if other factors are contributing to these bird deaths."
The American Bird Conservancy (abcbirds.org), the nation's leading bird conservation organization, has been wondering about this for several years. Its interest was piqued by a 2005 federal study titled "Interactions Between Migrating Birds and Offshore Oil and Gas Platforms in the Northern Gulf of Mexico." The study included concern for potentially large numbers of night-migrating birds that might become fatally attracted to illuminated oil and gas platforms.
On cloudy nights during migration, hundreds and sometimes thousands of birds experience a phenomenon called "nocturnal circulation." Birds confuse platform lights with the stars by which they navigate. The birds become reluctant or unable to escape the lights' effects. Some fly until exhausted and drop into the water. In some cases estimates of as many as 100,000 birds have circled a single platform.
Other times birds land on the platforms where they may rest for hours or even days. But without food or water, they eventually die when they make a last desperate effort to leave the platform. That's how land birds get into seawater where sharks happily gobble them up.